Was the choke seated/screwed in all the way? Did it have a dent in the bottom edge prior to use? Are chokes centered in that particular gun when seated?(look throught the choke and see if the lip is the same all the way around)
Very unusual for a load to grab the bottom lip of the choke if none of the above are present. I always screw chokes in as tight as I can by hand and peek down the bbl to see if it's centered.
